Powertrains having an internal combustion engine coupled to a continuously or infinitely variable transmission (CVT) may be employed to provide tractive effort in vehicles. A CVT is capable of operating at input/output speed ratios that are infinitely variable over a range between a minimum (underdrive) ratio and a maximum (overdrive) ratio, thus permitting selection of engine operation that achieves a preferred balance of fuel consumption and engine performance in response to an output torque request, which may originate from a vehicle driver. The capability of providing infinitely variable input/output speed ratios distinguishes a CVT from a step-gear transmission that employs a plurality of fixed gear and associated gear ratios that may be selectively engaged in response to an output torque request.
Known chain-type CVTs include two pulleys, each having two sheaves. A chain runs between the two pulleys, with the two sheaves of each of the pulleys sandwiching the chain therebetween. Frictional engagement between the sheaves of each pulley and the chain couples the chain to each of the pulleys to transfer torque from one pulley to the other. One of the pulleys may operate as a drive or input pulley, and the other pulley may operate as a driven or output pulley. The gear ratio is the ratio of the torque of the driven pulley to the torque of the drive pulley. The gear ratio may be changed by urging the two sheaves of one of the pulleys closer together and urging the two sheaves of the other pulley farther apart from each other, causing the chain to ride higher or lower on the respective pulley. The urging of the sheaves of the pulleys may be accomplished by applying controlled hydraulic pressure. Hydraulic pressure control routines may be underdamped, causing pressure overshoot events or overdamped, causing pressure undershoot events, both of which may result in system instability and reduced performance.